Um...drop it? I wouldn't bother with the apology, since apart from your juvenile comment about her not having a brain, you haven't done anything that I'd necessarily apologize for. Sounds like you're both about 14, in which case, if you can let it go and just not talk to her for a while, she'll forget about it eventually.
I have a hard time believing you're a university student if you're worried about whether or not apologizing to a girl will make her think you "like" her. It sounds very much like the "girls have cooties" comments I hear on my son's elementary school playground.
Let it go - you'll undoubtedly tick off many, many people in your lifetime, and if you obsess about them all, you'll give yourself a nervous breakdown. Just try to be nicer the next time you're thinking about telling someone they have no brain.
